This invention relates generally to a process for the treatment of oil-containing or oil-producing solids to extract fuel gases and liquid crude oil products therefrom. More particularly, the invention relates to a process for the retorting of oil shale to produce a high BTU product gas in addition to liquid shale oil and, at the same time, to recover as much heat energy from the retorted oil shale as is practicable while at the same time discharging a flue gas essentially free of sulfur compounds to the atmosphere.
Vast deposits of oil shale, a sedimentary inorganic rock containing about 35 weight-percent calcite (CaCO.sub.3), 15 weight-percent dolomite (MgCO.sub.3.CaCO.sub.3), and 10 weight-percent alkali metal salts are known to exist in the United States, especially in the Green River formation in Colorado, Utah, and Wyoming. The oil shale in these deposits contains between 5 and 35 weight-percent of hydrocarbons in a form known as kerogen. When pyrolized, this kerogen decomposes to produce crude shale oil vapors, which, upon condensation, become a valuable source of fuel.
Several pyrolytic processes have heretofore been developed to produce crude shale oil from oil shale. One such process is shown in my previous U.S. Pat. No. 3,361,644, which is incorporated herein by reference. In this process oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale continuously exchanges heat with a downwardly flowing high-specific-heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In the lower section (the preheating zone), the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier hydrocarbons condense in this lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain more product oil. The remaining gases are then utilized as a product gas, a recycle gas as hereinbefore described, and a fuel gas to heat the recycle gas to the hereinbefore specified temperature of 1200.degree. F.
The advantages of this process, especially in comparison to those processes wherein retorting heat is generated by combustion within the retort itself, and wherein a gas containing air is used as the combustion-eduction gas, are numerous. Firstly, the product gas is of high BTU content and is therefore suitable as a commercial fuel. Secondly, by using a high specific heat recycle gas, it is possible to educe more oil from the shale rock per volume of recycle gas utilized; thus, higher mass velocities of oil shale can be employed and no loss in yield is realized. Also, the use of a recycle gas containing essentially no oxygen avoids the oxidation and degradation of the shale oil product into gums, tars, etc. Furthermore, since the recycle gas is heated by means external to the retort, retorting temperature control difficulties (which usually result in excessive cracking of shale oil vapors in the retorting zone and the formation of clinkers which adversely affect the flow of the oil shale in the retort) are not encountered. Moreover, because of the better control of temperature in the retorting zone, the process can be so optimized that minimum heating rates, maximum oil yields, and a minimizing of the amount of coke left on the retorted shale can all be achieved. Lastly, no problem is encountered, as is common in prior art gas-upflow retorting processes, of refluxing of product oil in the preheating and eduction zones, with consequent loss of yield by polymerization into heavy residual fractions; instead, the condensed liquid product is continuously swept by gravity and gas flow away from the retorting zone.
However, one disadvantage in the foregoing process resides in the use of a portion of the product gas as fuel for heating the recycle gas, rather than using the coke on the spent shale. This represents a loss in thermal efficiency and a wasting of potential heat energy. Prior art attempts to use the coke in the retorted shale to provide heat energy for heating the recycle gas usually result in other disadvantages. For example, in U.S. Pat. No. 3,503,869 to Haddad, the use of the coke in retorted shale as a source of fuel necessarily results in a dilution of the product gas with gaseous products of combustion; this produces a product gas of lower BTU content than that produced in processes having effective means for separating the recycle gas and the flue gas. Thus, a method is required which will utilize the potential heat energy of the coke on the retorted shale without also sacrificing the advantages obtained in my previously described process.
In addition to the difficulties posed by the foregoing, the development of a practical shale oil recovery method is also hampered by the fact that the operation of present commercial processes results in the atmospheric discharge of flue gases containing excessive proportions of sulfur compounds sometimes in excess of 3000 ppmv total sulfur compounds. In the U.S.S.R., for example, one of the major impediments to the development of a successful shale oil recovery process is the difficulty in preventing the atmospheric discharge of sulfur compounds. (See Oil & Gas Journal, Vol. 73, No. 40, Oct. 6, 1975, pages 42-43).
A review of present oil shale retorting techniques will reveal that the discharge of sulfur compounds therefrom is especially difficult to prevent. In processes wherein a portion of the product gas is utilized as a fuel to provide heat for retorting purposes, the H.sub.2 S normally present in said fuel is also burned and is hence discharged to the atmosphere as SO.sub.2. In those processes wherein the coke on the retorted shale is burned to provide direct heat for retorting, the operating conditions are usually such that only partial combustion of the coke is effected, this being necessary to prevent temperatures in the combustion zone from becoming excessive and thus causing clinkering and unnecessary cracking of the shale oil vapors. In so doing, however, H.sub.2 S, a contaminant which some air pollution regulations require to be discharged in concentrations no greater than about 10 ppmv, is released from the coke in substantial amounts and must be removed from the flue gases by means of costly sulfur recovery processes. On the other hand, in those processes in which the coke is fully combusted, the resulting flue gases may contain excessive amounts of SO.sub.2, another pollutant whose atmospheric discharge must be controlled. In Colorado, for example, SO.sub.2 is required to be discharged in concentrations no greater than 150 ppmv, or no greater than 500 ppmv if the total amount of SO.sub.2 discharged in one day is no greater than 5 tons.
It is therefore one object of the present invention to provide an oil shale retorting process which significantly reduces atmospheric pollution caused by discharge of gaseous sulfur compounds. It is another object to provide a process combining the advantages of my process described in U.S. Pat. No. 3,361,644 with that of utilizing to the fullest extent possible the potential heat energy available in the coke in the retorted shale. It is yet another object to provide an oil shale retorting process whose overall efficiency is maximized by converting most of the kerogen in the oil shale to useful products of shale oil and undiluted, high BTU fuel gas, while the remainder is utilized to the fullest extent possible as a source of heat energy. It is another object to provide a method for continuously combusting essentially all the coke on the retorted shale traversing the combustion zone of an oil shale retorting process and, at the same time, continously removing from said combustion zone a flue gas that is essentially free of sulfur compounds. Other objects will appear to those skilled in the art from the specification and claims herein.